Ion implantation is a standard technique for introducing property-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the sub-surface of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity or material property.
In conventional beamline ion implantation processes, ions are provided toward a substrate in beams that may comprise generally parallel ions. Thus, for a given substrate orientation with respect to the ion beam principle axis, the ions impinge at the same angle of incidence. This facilitates control of implantation of ions into the substrate since for any given substrate orientation, the direction of ion implantation is well characterized. However, when multiple angle implantations are desired, it is necessary to move the substrate orientation, beam direction, or both. Moreover, the ion beam generally covers a wide area of the substrate, requiring masking in order to implant only desired areas.
Recently, techniques and apparatus have been developed to provide ions to a substrate over a range of angles. FIG. 1 is a block diagram that depicts a processing system that provides ions at multiple angles to a substrate. The processing system 10 includes a plasma source 12, an extraction plate 14 (or sheath engineering plate), and a process chamber 16. A gas source 18 is connected to the process chamber 16. The plasma source 12 or other components of the processing system 10 also may be connected to a pump (not shown), such as a turbo-pump. As illustrated, the plasma source 12 is an RF plasma source with an RF generator 20, an RF matching network 22, and antenna 23. The plasma source 12 is surrounded by an enclosure 24 and an insulator 26 separates the enclosure 24 from the process chamber 16. The process chamber 16, plasma source 12, or workpiece holder 28 may be grounded.
The extraction plate 14 is used to form ion beam 30 for implantation into a workpiece 40, when a bias is applied between plasma 32 and workpiece 40. The extraction plate 14 may be cooled. The plasma source 12 may be biased and a bias power supply 52 may be provided to provide a continuous or pulsed bias on the substrate with respect to the plasma 32 to attract the ion beam 30. The extraction plate 14 may have at least one aperture 34, through which ion beam 30 is provided to the workpiece 40. Additional description related processing systems can be found in co-pending U.S. patent application Ser. Nos. 12/417,929, filed Apr. 3, 2009, and issued as U.S. Pat. No. 7,767,977; Ser. Nos. 12/418,120, filed Apr. 3, 2010; 12/644,103, filed Dec. 22, 2009; and 12/848,354, filed Aug. 2, 2010, each of which is herein incorporated in its entirety by reference.
An ion beam 30 extracted from a plasma using processing system 10 may be used to simultaneously provide to workpiece 40 the plasma 32 over a range of angles if desired without requiring complicated masking or lithography procedures. This ability to create a wide angular distribution of ions facilitates processing of substrates having three dimensional features where it may be desirable to simultaneously provide ions incident on the features from different directions. Moreover, the exact angular distribution of ion beam 30 that are provided to workpiece 40 may be established according to a specific set of ion beam optics conditions (parameters) in processing system 10. Parameters that may affect the angular distribution of ion beam 30 include the shape and size of aperture 34, the implantation voltage (the voltage difference between plasma 32 and workpiece 40), spacing between extraction plate 14 and workpiece 40, and plasma density. Thus, a specific set of parameters may establish a specific ion angular distribution of ion beam 30.
However, because the angular distribution of ion beam 30 may be sensitive to such parameters as plasma density, the angular distribution may be subject to change over time as operating parameters or conditions within processing system 10 drift. Moreover, certain angular distributions of ions may not be achievable using processing system 10 according to any single set of control parameters of processing system 10.
In view of the above, it will be appreciated that it may be useful to provide improvements to control the angular distribution of ions in ion implantation systems.